I assume you stopped doing the quest chain when you unlocked the vendors. At some point you either abandoned or lost the quest "Forging an alliance" and now you can't see the vendor NPC.
Go back to Thorim (Temple of Storms, The Storm Peaks) and re-pickup the quest "Forging an alliance" (ID: 12924). After that you should be able to buy stuff from the vendor and continue the quest chain if you feel like it.
I also have the same issue where I can't find the NPC and I have done the quest chain up until the tower up that mountain and I didn't get any missions going forward.. And I also don't have missions to do in stromy peaks accept for the daily quests and my rep with sons of hodir is exalted... What can I do to get that NPC please